Insights into the Stability and Structures of Phosphine-Boranes and Their α‑Metalated Derivatives

The reaction between iPr2PCl and Ph2P­(BH3)­CH2Li gives the mixed phosphine/phosphine-borane
Ph2P­(BH3)­CH2PiPr2 (1a) in good yield. Thermolysis of 1a leads to borane migration and the formation of Ph2PCH2P­(BH3)iPr2 (2a) along with small amounts of Ph2P­(BH3)­CH2P­(BH3)iPr2 (3a) and Ph2PCH2PiPr2 (4a). Compound 3a may be synthesized directly
from the reaction of 1a with BH3·SMe2, while 4a can be prepared cleanly by heating 1a in methanol under reflux. Kinetic studies on the conversion
of 1a to 2a reveal the reaction to be apparently
first order in 1a, suggesting a dissociative process,
and yield the activation parameters ΔH⧧ = 63 ± 8 kJ mol–1, ΔS⧧ = −145 ± 24 J K–1 mol–1, and ΔG⧧ = 106 ± 8 kJ mol–1, the negative entropy
of activation conversely suggesting an associative process. DFT studies
suggest that concerted migration of borane within a molecule of 1a is disfavored, but that both the dissociative and associative
mechanisms for borane migration operate simultaneously. Metalation
of 1a–4a with nBuLi
in the presence of tmeda gives the complexes [{Ph2P­(BH3)}­CHPiPr2]­Li­(tmeda) (1b), [Ph2PCH­{P­(BH3)iPr2}]­Li­(tmeda) (2b), [{Ph2P­(BH3)}­CH­{P­(BH3)iPr2}]­Li­(tmeda) (3b), and [Ph2PCHPiPr2]­Li­(tmeda)
(4b), respectively, which adopt similar structures in
the solid state. Analysis of the crystal structures suggests that
the phosphine-borane groups stabilize the adjacent charge to a greater
extent than the phosphine groups. This is supported by DFT calculations,
which show that the greatest delocalization of negative charge from
the carbanion is into the P–C­(Ph) or P–C­(Pr) σ*-orbitals
of the phosphine-borane substituents.